Huh. Found a way of spotting vampires 100% of the time.
Numbered every single one of my migrant waves from the time they arrived, and never spotted a single vampire death. I screened each individual migrant from migrant to migrant, checking for oddities, with an exception to migrant wave 8.
This wave I segregated from the rest of the fort, and skimmed over without paying much interest to them at all. I used them for lycanthrope testing, where just a while ago I found a really strange oddity where myself, expecting the Dwarves I sent to die were saved by one particular Dwarf with no combat skills or experience savagely beat the ever hating crap out of the original were-civet. All Dwarves involved were bitten, shaken and bleeding like no tomorrow, confirming the curse had been passed on, and sure enough, come full moon, all of the Dwarves had transformed.
Except of course, the hero Dwarf. Shocked, I checked over that Dwarf and saw nothing suspicious - explaining how I never caught it in the first place, and low and behold, it was over 280 years old. Vampire confirmed. It was just my luck that I confined that one wave away from the entire fort... And I had a were-civet.
Science prevails